Seismic streamer formed of sections comprising a main sheath covered with an external sheath formed using a thermoplastic material loaded with a biocide material

ABSTRACT

Described herein is a seismic streamer formed of sections including a main sheath covered with an external sheath, wherein said external sheath is formed using a thermoplastic material loaded with a biocide material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of French Patent Application No. FR08/04282, filed on Jul. 28, 2008, which application is herebyincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the invention is that of the acquisition of seismic data.More precisely, the invention relates to equipment for analysing seabeds. The invention in particular relates to the industry of oilprospection via the seismic method, but can be applied to any fieldimplementing a seismic data acquisition network in a marine environment.

2. Background of the Invention

The operations of acquiring, in the field, geophysical data,conventionally implement networks of sensors. These sensors are mostoften hydrophones; however said operations can also implement geophones,accelerometers or any other type of sensors. The hydrophones aredistributed along cables in order to form linear acoustic antennascommonly designated by the terms “streamers” or “seismic streamers”. Thenetwork of seismic streamers is drawn by a seismic boat.

A seismic streamer is comprised of an assembly of individual sectionsinstrumented with seismic sensors and associated digital-analogueconversion electronics. The seismic boat draws also one or severalseismic sources comprised of an air gun network, water guns or acousticvibrators. The pressure wave generated by the seismic source crosses thehead of water and insonifies the upper layers of the sea bed. A portionof the signal is refracted by the interfaces and the inhomogeneities ofthe oceanic crust. The resulting acoustic signals are then detected bythe seismic sensors distributed across the entire length of the seismicstreamers. These acoustic signals are conditioned, digitised andretransmitted by the telemetry of the seismic streamers to the operatorstation located on the seismic boat where the raw data processing iscarried out.

Seismic exploration campaigns can be scheduled over periods extendingover several months. In addition, even in the case of unfavourableweather conditions, it is avoided, as much as possible, to return thestreamers to the vessel, as the operations of returning/deploying are inpractice particularly tedious and long. Indeed, each streamer has alength of several kilometers (up to 12 kilometers). This results in thatseismic streamers can remain immersed in sea water for severalconsecutive months (with period of immersion of 6 to 12 months beingfrequent). Moreover, seismic streamers are generally immersed at ashallow depth (a few meters), and are generally dragged at low speed(less than or equal to 5 knots). Therefore, the tubular structure of thestreamers is subject to fouling, in particular due to the proliferation,on the external layer of the seismic streamers, of microorganisms orbio-fouling.

However, these bio-fouling generate in time several disadvantages, amongwhich:

-   -   they generate hydrodynamic flow noise: the proliferation of the        bio-fouling can result in the formation on the external layer of        the seismic streamers of amalgams or incrustations likely to        disturb the measurements to be carried out;    -   they tend to increase the drag of seismic streamers and,        consequently, the consumption in fuel of the boat that is        tugging them;    -   during the winding of seismic streamers on the winches of the        tug boat, the bio-fouling is deposited and fouls the equipment,        and generate, after a few days, a stench due to their        degradation in the air;    -   certain types of bio-fouling have a shell that can pierce the        sheaths of the seismic streamers, which can give rise to water        intake inside the seismic streamer, or to an oil leak (in the        case of a seismic streamer incorporating a filling fluid, such        as kerosene).

Several techniques have been proposed in prior art in order to overcomethe disadvantages generated by the proliferation of bio-fouling on theexternal sheath of the seismic streamers.

A technique is in particular known disclosed by patent FR-2 847 987,according to which it is proposed a cleaning device of the externalsheath of seismic streamers, which includes:

-   -   a tool for treating the seismic streamer, comprising rotary        brushes, as well as blades to abrade the incrustations formed on        the external sheath of the streamer;    -   means for positioning and guiding the device along the streamer.

The structure of such a device is relatively complex, in the sense wherethe design of the latter can be provided in order to allow for itspassage on protruding elements on the tubular structure of the seismicstreamers, these elements able to be comprised of floaters, or oftrajectory correction devices (commonly designated by the term “bird”).Such devices have been experimented with, and then abandoned inpractice, due to their cost and especially due to the fact that, most ofthe time, become separated from the seismic streamers and become lost inthe sea beds.

“Anti-fouling” paints are also known, conventionally used on boat hulls.However, “anti-fouling” paints generally contain TBT (tributyltin) whichis now well known for its toxicity. Such paints are thereforeunacceptable from an environmental standpoint (pollution of sea water)as well as in relation to the safety of persons likely to be in contactwith TBT paint during the manipulation of seismic streamers.Furthermore, the techniques for applying “anti-fouling” paint are hardlycompatible with the technical and economical constraints linked withseismic streamers. Indeed, the techniques for applying these paintsimply an operation of projecting paint onto the support. This operationof projection can be carried out by operators, using a projection gun.In this case, the projection operation is particularly long and costlydue to the high number of kilometers of streamers to be treated. Inaddition, it is necessary to comply with a drying time for the paint,which implies storing the seismic streamers in positions that take up aconsiderable amount of space in order to avoid any contact of theportions of treated streamers with themselves (which excludes thewinding of seismic streamers); such storage is consequently not viableeconomically.

It has also been proposed to carry out the projection of “anti-fouling”paint using projection rings, inside of which the tubular structure ofthe seismic streamers circulates, this with a paint having the capacityof drying dry in contact with water. The principle in implementing thistechnique consists in mounting the projection rings on the streamertugging vessel and in carrying out the operation of treatment usingrings in the open sea, the seismic streamers being stored in the wateras they are treated. Such a technique thus makes it possible to overcomethe problems of storage of seismic streamers during the drying phase ofthe paint, but implies very high implementation costs. Indeed, theequipment costs for boats are substantial in the sense where it isnecessary to provide as many projection rings as there are winches forwinding seismic streamers (up to twelve winches can be mounted on aseismic prospection boat). Furthermore, the logistics and means forstorage of the paint are added to the other equipment present on theboat, which is already of a substantial number.

SUMMARY OF THE INVENTION

The invention in particular has for objective to overcome thesedisadvantages of prior art. More precisely, the invention has forobjective to propose a technique making it possible for a seismicstreamer to resist the fouling by bio-fouling, that can be implementedusing industrial processes that are in particular less costly comparedto the techniques mentioned in reference to prior art. The inventionalso has for objective to provide such a technique that is compatiblewith the mechanical constraints linked with seismic streamers, inparticular in relation to their capacity of being wound. The inventionalso has for objective to provide such a technique that is compatiblewith the internal components of seismic streamers, entailing inparticular components transporting an electrical current. The inventionalso has for objective to provide such a technique which avoids asubstantial increase in weight of the seismic streamers.

These objectives, as well as others that shall appear in what follows,are achieved thanks to the invention which has for purpose a seismicstreamer of the type comprising sections including a main sheath coveredwith an external sheath, characterised in that said external sheath isformed using a thermoplastic material loaded with a biocide material.

As such, thanks to the invention, the sections comprising the tubularelements of the seismic streamers are treated against the proliferationof bio-fouling on the external layer of these sections, the biocidematerial embedded in the thermoplastic material of the external sheathbeing distributed progressively and continuously on the surface of theexternal sheath. In other words, the thermoplastic material of theexternal sheath is permeable to the biocide material of which it isloaded.

It is understood that the invention of course allows the marinemicroorganisms to attach themselves on the external sheath of theseismic streamers, but also that the treatment according to theinvention makes it possible to eliminate them continuously thanks to thebiocide effect of the corresponding material of which is loaded theexternal sheath of the streamer.

Moreover, the invention proposes to cover the main sheath with a genuinesheath, not just a simple coat of “anti-fouling” paint as mentioned inreference to prior art. This results in that this sheath, made from athermoplastic material, allows recourse to proven manufacturingtechniques and which are economically viable, such as co-extrusion orbi-extrusion.

As such, the invention allows for the use of current techniques in thefield of plastics transformation, avoiding the problems encountered withthe application of “antifouling” paint, in particular entailing:

-   -   treating very long sections of seismic streamers, this using        industrial processes that limit human intervention;    -   avoiding the immobilisation of the sections of seismic        streamers, and therefore their storage, due to the fact of        complying with the drying time;    -   avoiding the implementation of additional equipment on the        seismic streamer tug boats;    -   suppressing recourse to toxic substances.

The design of the external sheath, in a thermoplastic material, allows,as mentioned hereinabove, the implementation of extrusion techniquesthat group together the advantages:

-   -   easily mix granules of thermoplastic materials with a biocide        material, upstream of the heating-extrusion operation;    -   be particularly adapted to the carrying out of linear products        of possibly long lengths;    -   limit the storage constraints after extrusion;    -   allow the sheathing of the sections of seismic streamers on        dedicated sites independent of the tug boats;    -   allow for the treatment of sections of seismic streamers        according to the invention at costs that are particularly        reduced in relation to the techniques of prior art.

In addition, the external sheath exerting the biocide effect is obtainedwithout using solvents contrary to “anti-fouling” paints, which is ofcourse an advantage from an environmental standpoint.

According to a preferred embodiment, said biocide material includescopper metal. It has indeed been shown that copper has a toxicity forphytoplankton and other marine microorganisms, without however causingtoxicity for man and without generating, or hardly, any marinepollution. Note that copper exerts a biocide effect once in contact withwater, due to its oxidation. According to an advantageous solution, saidexternal sheath includes between 75 and 85% copper metal. In this way,the biocide effect of the external sheath is optimised, while stillavoiding the external sheath from becoming a conductor of electricity.Preventing the external sheath from being conductive of electricityavoids any problem of galvanic coupling between the external sheath andthe main sheath. Such a galvanic coupling would tend to inhibit theoxidising power of the biocide material and, consequently, the capacityof the streamer to oppose the proliferation of microorganisms.

According to another embodiment, said biocide material comprisespowdered silver. Such an embodiment is also effective, but generateshigher costs than recourse to a copper metal. Note that silver exerts abiocide effect once in contact with water, due to an electrolyticreaction.

Advantageously, said external sheath has a thickness of less than 1 mm.In this way, said external sheath has a thickness that is limited butthat is sufficient to contain a quantity of biocide material exertingthe expected effect, this while still preventing from conferring anadditional rigidity to the seismic streamer, which would be detrimentalto its capacity of being wound on a winch. Furthermore, an externalsheath according to the invention having such thicknesses makes itpossible to avoid increasing the weight of the seismic streamerexcessively.

According to a first alternative embodiment, said main sheath and saidexternal sheath include the same thermoplastic material, with thesections of seismic streamers able to be obtained in this case by amethod of manufacturing according to which the external sheath and themain sheath are carried out during a step of co-extrusion.

According to a second alternative embodiment, said main sheath and saidexternal sheath include different thermoplastic materials, a binderbeing disposed between said main sheath and said external sheath, withthe sections of seismic streamers able to be obtained in this case by amethod of manufacturing comprising:

-   -   a step of extrusion of said main sheath;    -   a step of depositing a binder on said main sheath;    -   a step of extrusion of said external sheath on said main sheath        coated with said binder.

Other characteristics and advantages of the invention shall appear moreclearly when reading the following description of two preferredembodiments of the invention, and of several of its alternatives,provided by way of examples for the purposes of information and whichare non limiting, and annexed drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a cross-section view of the various layers and components of asection of seismic streamer according to a first embodiment of theinvention;

FIGS. 2 and 3 are views respectively of a cross-section and inperspective of the main and external sheaths of a section of seismicstreamer according to the invention.

DETAILED DESCRIPTION

The principle of the invention resides in the proposing of a seismicstreamer wherein sections having a protection against bio-fouling, thisprotection taking the form of an external sheath added onto the mainsheath of the sections, this external sheath being formed using athermoplastic material loaded with a biocide material.

In reference to FIGS. 2 and 3, a section (or tubular element) of aseismic streamer includes a main sheath 1 delimiting a body wherein aremounted hydrophones, power cables and data transmission cables. Anexternal sheath 2 is added to the main sheath 1 in such a way as to coatthe latter.

Note that a section of seismic streamer generally has a length of 150meters, the seismic streamer able to have a total length ofapproximately 12 kilometers.

Within the framework of the invention, the seismic streamer can beindifferently of the “fluid” type or “solid” type or any other seismicstreamer filling technology such as gel. Recall that a seismic streamerof the “fluid” type incorporates in the main sheath kerosene of whichthe function is double: a density correction function in such a way thatthe density of the streamer is neutral in sea water (i.e. the streamerdoes not sink nor does it float or, in other words, the streamer and thesea water have the same density), and an acoustic function (the keroseneplaying the role for the transmission of waves). A seismic streamer ofthe “solid” type is in particular differentiated from a seismic streamerof the “fluid” type in that it incorporates in the main sheath animpermeable foam (foam with closed cells) instead of kerosene.

FIG. 1 shows, as a cross-section view, a section of seismic streamer ofthe “solid” type. According to this embodiment, such a section includes:

-   -   an external sheath 2;    -   a main sheath 1;    -   an annular volume of foam with closed cells 3 (allowing for the        control of the floatability of the streamer);    -   a plurality of telemetry cables 4;    -   a layer 5, providing the maintaining of the telemetry cables 4;    -   a tubular element made of Kevlar© 7, providing the mechanical        resistance of the streamer by taking up the traction efforts;    -   electrical power cables 8.

According to the principle of the invention, the external sheath 2 isformed using a thermoplastic material loaded with a biocide material.According to this embodiment, the biocide material is comprised of thepowdered copper metal mixed with granules of thermoplastic material, thepowdered copper metal being intended to be embedded in the thermoplasticmaterial after fusion of the latter during a step of heating-extrusion.In addition, the external sheath comprises approximately between 75% and85% of copper metal, which corresponds to a percentage that is optimisedin order to produce the expected biocide effect while still preventingthe external sheath from becoming a conductor of electricity. Note thatanother biocide material can be incorporated into the external sheathaccording to other embodiments that can be considered, such as powderedsilver.

Preferentially, the external sheath 2 has a thickness of less than 1 mm,the main sheath having a thickness of between 3 mm and 5 mm. Note thatthis thickness of the main sheath is selected in order to sufficientmechanical protection of the various cables and components of theseismic streamer. As such, the invention makes it possible to limit theincrease in rigidity and in the weight of the seismic streamer by addingto the main sheath a thin external sheath, rather than incorporating thebiocide material comprised of a copper metal or of powdered silverdirectly in the main sheath, which would result, in light of therelatively substantial thickness of the main sheath, in a considerableincrease in the rigidity and in the weight of the seismic streamer.

According to the embodiment shown in FIG. 1, the main sheath 1 and theexternal sheath 2 include the same thermoplastic material, herepolypropylene, or polyurethane or a polyamide. According to thisembodiment, the main sheath and the external sheath are carried out byco-extrusion.

According to the embodiment shown in FIG. 2, the main sheath and theexternal sheath include different thermoplastic materials, in the groupof the following materials: polypropylene, polyurethane or polyamide.Furthermore, a binder can be disposed between the main sheath and theexternal sheath. In this case, the method of manufacturing of thecorresponding section of seismic streamers includes the steps of:

-   -   extrusion of the main sheath;    -   depositing binder on the main sheath;    -   extrusion of the external sheath on the main sheath coated with        the binder.

It is understood that in one or the other of the embodiments that havejust been described, the external sheath is adhered to the main sheath(by the intermediary, or not, of a binder), which avoids any laterrelative displacement of the external sheath and of the main sheath.

Note that after a certain period of use of a seismic streamer accordingto the invention, a maintenance operation may be necessary in order toreactivate the biocide effect of the corresponding material incorporatedinto the external sheath. For this, a simple mechanical attack of theexternal sheath suffices, this mechanical attack able to be carried outusing a high-pressure cleaner or via a brushing technique.

The invention claimed is:
 1. A seismic streamer comprising sectionsincluding: a main sheath formed using a thermoplastic material, anexternal sheath, formed using a thermoplastic material loaded with abiocide material, and a binder being disposed between the main sheathand the external sheath, wherein the main sheath is covered with theexternal sheath, wherein the main sheath and the external sheath includedifferent thermoplastic materials, wherein the main sheath is formed byextrusion, wherein the binder is deposited on the main sheath, andwherein the external sheath is extruded on the main sheath coated withthe binder, wherein the external sheath is adhered to the main sheath bythe binder.
 2. The seismic streamer according to claim 1, wherein thebiocide material includes copper metal.
 3. The seismic streameraccording to claim 2, wherein the external sheath includes between 75%and 85% copper metal.
 4. The seismic streamer according to claim 1,wherein the biocide material includes powdered silver.
 5. The seismicstreamer according to claim 1, wherein the external sheath has athickness of less than 1 mm.
 6. The seismic streamer according to claim1, wherein the main sheath has a thickness between 3 mm and 5 mm.
 7. Amethod of manufacturing a section of a seismic streamer, wherein thesection comprises: a main sheath formed using a thermoplastic materialand an external sheath formed using a thermoplastic material loaded witha biocide material, the main sheath and the external sheath includingdifferent thermoplastic materials, wherein the method comprises: a stepof extrusion of the main sheath, a step of depositing a binder on themain sheath, and a step of extruding the external sheath on the mainsheath coated with the binder, wherein the external sheath is adhered tothe main sheath by the binder.